Visa Expiry

I am a UK citizen and I currently have a three month visa which was issued on 2nd September. I arrived in Libya on the 5th September. I work with another person whose visa was issued at the same time but who arrived on 14th September. We both need to renew our visas soon but cannot go back at the same time. What I wish to know is does the visa last for three months from the date of issue or from the date of arrival?

If it is a 3 month multiple it should be 90 days from date of stamping in the passport.